6 Attachment(s)
Of planes & adjustment systems
There’s a bit of a roundabout story to set the scene for this post, so bear with me.
Some years ago, I assembled a Lee Valley plane kit & wrote it up as a review for Aust, Wood Review (Issue 84, page 32): Attachment 456696
I followed instructions to make the wedge/lever cap, but wasn’t very happy with it, the thin wooden wedge was a bit flimsy, imo, so I tried modifying it & posted on it here.
While my ‘improvement’ worked a bit better, it still had some issues and so I ended up replacing it with a brass lever cap. That sorted things much better, but still that plane just never became a favourite of mine. For a start, being a ‘single iron’ plane, it struggled with really wild wood, & I also found it uncomfortable to use because the rear ‘bun’ was too fat for my hands, but being a laminated construction, I couldn’t chop it down more because it would have cut away much of the glueing area between sides & core. One day, I decided to decommission it, & re-purposed the lever cap for my little rear-bun smoother, which turned out to be a far sweeter thing all round: Attachment 456690
I didn’t want to waste the LV bits, so I’ve been plotting & planning one more infill……
Now, for reasons only nutters like me comprehend, I want to go the full hog on this plane & give it a rear handle rather than a bun. That creates a major problem. The adjuster mechanism that comes with the kit is from their stock-standard block plane. As such, it’s quite short, and quite unsuited to using with a handle that would fit anyone with a normal-sized hand. Note the position it would occupy when set against my handle template. To use the shaft provided, I would have to cut a deep notch in the upper part of the handle, which would make the top rather thin & fragile: Attachment 456693
The only solution to this I could see was to make a new, longer shaft, which was a bit more bother than I first thought, because it requires a 3/16 N.F. left-handed thread on the end that goes into the spigot that engages the blade. I happened to have a nice bit of shafting (out of a dead inkjet printer) but of course I don’t have any L.H. dies, so I had to learn to cut the thread on my little lathe, which turned out not to be as fearsomely difficult as I’d imagined, once I worked out that the manual gave the wrong combination of cogs to cut 32 tpi! On the 3rd try, I got a useable L.H. thread that worked nicely. The other thread, which engages the pivot-point, was easy, it is ¼” N.F. (right-handed), & I have a tap & die set for that size. So after a fun afternoon’s work, I have my desired long adjuster shaft: Attachment 456692
OK, so that part of my projected new plane is taken care of, but in sorting all this out, I had an opportunity to think about the “Norris” adjuster and its mysteries a bit more. My first ‘discovery’ was that for about 40 years I’d been under a complete misapprehension about how they work. Somewhere, way back, I read someone’s explanation of Tom Norris’s patented mechanism. For those who don’t know what the guts of a Norris adjuster looks like, it consists of two parts, an outer shaft threaded through a pivot point, and an inner shaft that is attached to the ‘banjo’ which engages the cap-iron screw. The inner shaft has a finer, left hand thread. This is a pic of my late-model A5 (it’s not possible to remove it for a better pic because I would have to remove the riveted-in cap-iron, & I’m not about to do that: Attachment 456695
Now the explanation I read suggested that as the shaft is turned, the inner shaft screws in the opposite direction (because it is a LH thread) and counteracts the other thread, so that the banjo moves forward & backward at a rate determined by the difference between thread counts of the outer (32 tpi on earlier models, 35 on later), and inner (40 tpi on all models). This is patently wrong, and had I spent a few minutes reflecting on it at the time I should have seen it was so. In fact, the effect of the double thread is to move the banjo fore & aft at a speed which is the sum of the two threads, not the difference, and that is the source of many complaints from Norris users that the adjustment is too coarse.
Veritas, in their wisdom, copied the same R.H./L.H. configuration for their adjusters, but use a solid shaft inbstead of the sleeved arrangement of Norris's (far simpler to produce). The one in question here has threads of 20 and 32 tpi respectively. You can check my arithmetic, but it works out that the 20 tpi thread moves the spigot .05” each turn, while the 32tpi thread adds another .03125” for a total movement of 0.08” each turn. I calculated the Norris’s movement, and it is a net of 0.0575” per turn, a little less. But that's still a whopping 2mm (LV) or 1.2mm (Norris) which is compounded by the tendency of this style of adjuster to slew the blade if it’s at all off-centre (which it usually is, unless you are capable of sharpening your blades with micro-precision!). No wonder I always struggle with setting either, & would rather use my adjusterless planes & a hammer!
So I got to thinking, which is often a dangerous occupation for me, & I wondered what would happen if I used two threads of the same spiral direction? It would mean that the second part (the banjo or spigot that engages the blade) would move at a rate that was the difference between the two tpi, not he sum. And just for fun, I made up an adjuster shaft, with the same threads as the LV adjuster, but both R.H. threads (i.e., 20tpi & 32tpi). The result is that the spigot now moves at a rate that reflects the difference in tpi. To put that into figures, where the LV adjuster moves the blade spigot ~2mm per turn, having both threads follow the same helical direction moves it 0.457mm per turn.
You will have to wait for a while for a report on the effectiveness of this arrangement because the plane to which it will be fitted is still on the drawing-board. The only drawback I can see so far is that I need to experiment with the lengths of the two threads a bit more, I made them equal on the prototype, but clearly, the coarse threaded section needs to be longer. No probs, when it comes time to fit it, I’ll alter the proportions of the two.
So why not just have a single, fine thread for these things? I really don’t know the answer to that, other than that a fine thread is a bit more fragile & maybe prone to wear & rapidly-developing backlash, but that’s purely a guess. If anyone can enlighten me, please do! Mr. Norris added an elegant feature to his creation by partially splitting the pivot nut (the one that takes the outer thread). A screw (which is clearly visible in the pic) tightens the split & can reduce backlash in that thread to zero (the inner thread remains susceptible to wear, however).
The Australian-made kit plane I put together a couple of years ago has a single thread adjuster. At 24tpi it’s not terribly fine, and it’s not an easy plane to adjust finely, partly for that reason & partly because the lever-cap screw is at a fairly acute angle to the blade & slews it a bit as you tighten it.
So here are three different versions of the so-called “Norris adjuster”, a simple, single thread of the kit plane in front, and the rear two with two different pitched threads:Attachment 456691
The one in the centre has left & right hand threads which produce the same effect as Norris’s sleeved shaft, and the one at the back has R.H. threads on both sections, which greatly reduce the rate of travel of the spigot. All will work, after their fashion, but which do you think is ‘best’??